This Heli-Max Axe Micro CX model helicopter is an example of a micro-sized coaxial model. Note the size comparison with the cellular telephone at right.

A Radio-controlled helicopter (also RC helicopter) is model aircraft which is distinct from a RC airplane because of the differences in construction, aerodynamics, and flight training. Several basic designs of RC helicopters exist, of which some (such as those with collective pitch control) are more maneuverable than others. The more maneuverable designs are often harder to fly, but benefit from greater aerobatic capabilities.[1]

Flight controls allow pilots to control the collective (or throttle, on fixed pitch helicopters), the cyclic controls (pitch and roll), and the tail rotor (yaw).[2] Controlling these in unison enables the helicopter to perform the same maneuvers as full-sized helicopters, such as hovering and backwards flight, and many that full-sized helicopters cannot, such as inverted flight (where collective pitch control provides negative blade pitch to hold heli up inverted, and pitch/yaw controls must be reversed by pilot).[3]

The various helicopter controls are effected by means of small servo motors, commonly known as servos. A solid-state gyroscope sensor is typically used on the tail rotor (yaw) control to counter wind- and torque-reaction-induced tail movement.[4] Most newer helicopters have gyro-stabilization on the other 2 axes of rotation (pitch and roll) as well. Such 3-axis gyro is typically called a flybarless controller, so-called because it eliminates the need for a mechanical flybar.[5]

The engines typically used to be methanol-powered two-stroke motors, but electric brushless motors combined with a high-performance lithium polymer battery (LiPo) are now more common and provide improved efficiency, performance, and lifespan compared to brushed motors, while decreasing prices bring them within reach of hobbyists. Gasoline and jet turbine engines are also used.[6]

Just like full sized helicopters, model helicopter rotors turn at high speeds and can cause severe injuries. Several deaths have occurred as recently as 2013.

Common power sources of remote control helicopters are glow fuel (also called nitro fuel, nitromethane-methanol), electric batteries, gasoline (petrol) and turbine engines. For the first 40 years, glow fuel helicopters were the most common type produced. However, in the last 10 years, electric powered helicopters have matured to a point where power and flight times have equaled glow fuel helicopters.

There have been two main types of systems to control the main rotors, mechanical mixing and cyclic/collective pitch mixing (CCPM). Most earlier helicopters used mechanical mixing. Today, nearly all R/C helicopter use CCPM.

Practical electric helicopters are a recent development but have rapidly developed and become more common, overtaking glow fuel helicopters in common use. Turbine helicopters are also increasing in popularity, although the high cost puts them out of reach of most people.

Glow fuel, or nitro fuel helicopters (not to be confused with gas, or gasoline powered helicopters) have been made in several sizes over the years. These are referred to as the "class" of the helicopter. They include 1/2A class, 15 class, 30 class, 50 class, 60 class and 90 class. These class numbers originated from the size of engine (engine displacement measured in cubic inches) used in the different models. For example, a helicopter with a 0.30 cu in (4.9 cm3) engine is a 30 class and a helicopter with a 0.90 cu in (14.7 cm3) engine was referred to as a 90 class helicopter. The bigger and more powerful the engine, the larger the main rotor blade that it can turn and hence the bigger the aircraft overall. Typical flight time for nitro helicopters is 7–15 minutes depending on the engine size and tuning. The maximum height of operation for RC helicopters, be it glow fuel, gasoline, turbine or electric, is effectively limited to the height at which the model is still visible. Most quality radio control systems have a range of over a mile, when the model would be long out of sight.

Two small electric helicopters emerged in the mid-1990s. These were the Kalt Whisper and the Kyosho EP Concept, flying on 7–8 × 1.2 Ah NiCad batteries with brushed motors. However, the 540-sized brushed-motors were on the limit of current draw, often 20–25 amps on the more powerful motors, hence brush and commutator problems were common.

Recent advancements in battery technology are making electric flying more feasible in terms of flying time. Lithium polymer (LiPo) batteries are able to provide the high current required for high performance aerobatics while still remaining very light. Typical flight times are 4–12 minutes depending on the flying style and battery capacity.

In the past electric helicopters were used mainly indoors due to the small size and lack of fumes. Larger electric helicopters suitable for outdoor flight and advanced aerobatics have become a reality over the last few years and have become very popular. Their quietness has made them very popular for flying sites close to residential areas and in places such as Germany where there are strict noise restrictions. Nitro helicopters have also been converted to electric power by commercial and homemade kits.

The smallest remote-controlled production model helicopter made (Guinness World Records 2006) is the Picooz Extreme MX-1 sold at many toy stores (although this is infrared controlled, not radio), electronics stores and internet stores, costing about $30 (£28). The next smallest is the standard Picooz helicopter.

Several models are in contention for the title of the smallest non-production remote-controlled helicopter, including the Pixelito family of micro helicopters, the Proxflyer family, and the Micro flying robot.

A recent innovation is that of coaxial electric helicopters. The system's simple direction control and freedom from torque induced yaw have, in recent years, made it a good candidate on small models for beginner and/or indoor use. Models of this type, as in the case of a full-scale helicopter, eliminate rotational torque and can have extremely quick control response, both of which are very pronounced in a CCPM model. Most cheaper models do not have a swashplate, but instead use a third rotor on the tail to provide pitch control. These helicopters have no roll control and have limited mobility.

While a coaxial model is very stable and can be flown indoors even in tight quarters, such a helicopter has limited forward speed, especially outdoors. Most models are fixed-pitch, i.e. the collective pitch of the blades cannot be controlled, plus the cyclic control is only applied to the lower rotor. Compensating for even the slightest breeze causes the model to climb rather than to fly forward even with full application of cyclic. More advanced coaxial constructions with two swash plates and/or pitch control (common for full scale coaxial helicopters like Kamovs) have been realized as models in individual projects but have not seen the mass market as of 2014[update].

More recently, multirotor designs have become popular in both the RC hobby and unmanned aerial vehicle (UAV) research. These vehicles use an electronic control system and electronic sensors to stabilize the aircraft. Multirotors are generally more affordable, easier to construct, and simpler to operate than RC helicopters. This made multirotor aircraft an appealing platform for amateur model aircraft projects and aerial photography.[8][9]

Small fixed-pitch helicopters need a 4-channel radio (throttle, elevator, aileron, rudder), although micro helicopters that utilize a 2-channel infrared control system also exist; while collective-pitch models need a minimum of 5 channels with 6 being most common (throttle, collective pitch, elevator, aileron, rudder and gyro gain). Because of the normal interaction of the various control mechanisms, advanced radios include adjustable mixing functions, such as throttle/collective and throttle/rudder.[10] Radio prices vary from $50–$3,000 USD.

Well-known manufacturers of helicopter-specific radio controllers include: Futaba, JR, Spektrum, Hitec, Sanwa (known as "Airtronics" in North America), Multiplex (a division of Hitec), and OrangeRX. The original preferred user interface for helicopter-oriented RC transmitters at the beginning of the RC helicopter hobby, from the early 1970s through about 1990, was the so-called "single-stick" or "knobby" style of multi-channel RC transmitter, possessing a single primary two-axis joystick with a special rotatable, self-centering knob atop the single joystick's shaft for all three of the helicopter's aerodynamic controls, combined into only one primary control mechanism. The horizontal/vertical joystick movements of such a joystick provide cyclic control, and the knob is used for operating the tail rotor control. Such radios became unavailable as factory-built new units at the start of the 1990s, but newer units are still made by RC-flying electronics hobbyists well into the 21st century in North America for their own personal use, for flying both RC helicopters and fixed-wing RC model aircraft.

Spektrum DX6i 6-channel spread spectrum computerized aircraft radio which may be used for both helicopters and fixed-wing models

Systems such as frequency-hopping spread spectrum (FHSS) used by Futaba employ frequency hopping on the 2.4 GHz band instead of the various frequencies in the lower MHz ranges. The advantage is that radios are no longer using a fixed frequency during flight, mitigating the risk of interference on that fixed frequency.

Systems such as Spektrum and JR use the DSM2 direct-sequence spread spectrum (DSSS) method, where they transmit on a pair of fixed channels chosen when the radio and receiver are turned on. Any subsequent systems would avoid using these channels and continue searching for another unused pair of channels.

With either method many radios can be transmitting at once without interfering with each other. The Futaba systems change frequency approximately every two milliseconds, so even if two transmitters are using the same channel they are not doing so for long. The pilot will not notice any abnormal behavior of the model in the 1/500th of a second that they are interfering. This gives one the advantage of turning on a transmitter without regard to channels currently in use by other pilots' radios.

One downside to 2.4 GHz is that precautions must be taken during installation since certain materials such as carbon fiber can mask the signal. In some cases, satellite receivers with secondary antennas need to be used to maintain better line-of-sight with the transmitter radio. Another drawback is that a 2.4 GHz standard has yet to evolve so that receivers and transmitters can be mixed regardless of their respective manufacturer.

Learning to fly a collective pitch RC helicopter takes time and practice. Many modelers join a club so they can be instructed by experienced RC pilots, or follow on-line guides.[11]

RC Helicopters usually have at least four controls: roll - cyclic pitch, elevator (fore-aft cyclic pitch), rudder (yaw) and pitch/throttle (collective pitch/power).[3] For simple flight, the radio is usually configured such that pitch is around −1 degree at 0% throttle stick, and somewhere around 10 degrees at 100% throttle stick. It is also necessary to modulate the throttle in conjunction with the pitch so that the model maintains a constant rotor speed. This is beneficial for consistent and smooth flight performance.

If aerobatic '3D' performance is desired, then automatic throttle, or idle up, mode of flight is used. In this mode, the collective pitch ranges from its negative limit at 0% throttle stick input, up to its positive limit at 100% throttle stick. The throttle, on the other hand, is modulated automatically to maintain a constant rotor speed and is usually at its lowest value when the throttle stick is centered and the pitch is 0. This mode allows the rotor to produce a thrust upwards (by using negative pitch) which, when the model is inverted, allows sustained inverted flight. Usually a more advanced computer radio is used for this kind of flying, which allows customization of the throttle-collective mix.

The cyclic and yaw controls are not by definition different in these two modes, though 3D pilots may configure their models to be much more responsive.

Construction is typically of plastic, glass-reinforced plastic, aluminium or carbon fiber. Rotor blades are typically made of wood, fiberglass or carbon fiber. Models are typically purchased in kit form from one of about a dozen popular manufacturers and take 5 to 20 hours to completely assemble.

These model helicopters contain many moving parts analogous to those on full-size helicopters, from the swashplate to rotor and everything in between.

The construction of helicopters has to be more precise than for fixed-wing model aircraft, because helicopters are susceptible to even the smallest of vibrations, which can cause problems when the helicopter is in flight.

Additionally, the small size and low weight of R/C helicopters and their components means that control inputs, especially cyclic (pitch and roll) can have a very fast response, and cause a rotation rate much faster than the equivalent input might produce on a full-size aircraft. This quick response can make the model unnecessarily difficult to fly. For this reason, most model helicopters either have a flybar or electronic stabilizing equipment.

Aerobatic helicopter flying has historically followed the Fédération Aéronautique Internationale rules, which for helicopters are labelled F3C. These include a predetermined routine of hovering and aerobatics.

An advanced form of RC helicopter flying is called 3D. During 3D flying, helicopters perform advanced aerobatics, sometimes in a freestyle form, or in a predetermined set of moves drawn up by the organisers of the competition. There are a number of 3D competitions around the world, two of the best-known being the 3D Masters in the UK and the eXtreme Flight Championship (XFC) in the USA.

In 2008, the Fédération Aéronautique Internationale introduced the class of F3N as a provisional class for international 3D competition and in 2010 at the CIAM Plenary meeting, F3N gained formal approval for competition effective 1st January 2011.

F3N rules are designed to provide a consistent standard of judging throughout the World and give countries the chance to field a Team at a World championship hosted every two years. F3N is conducted in a similar manner as 3D Masters and 3DX with 3 round types made up of Set Manoeuvres, Freestyle Flight and Flight to Music.

While some companies make use of RC multicopters for low altitude aerial photography, filming, policing, and remote observation or inspection, RC helicopters are not commonly used for commercial purposes.

US Federal Aviation Administration regulations from 2006 grounding all commercial RC model and unmanned aerial vehicle (UAV) flights have been upgraded to require formal FAA certification before being permitted to fly at any altitude in the United States.[12] This move is being legally challenged.[13] Commercial drone/helicopter flights can be authorised, under restrictions, on a case by case basis through FAA certificates of authorization (COA). Since 2007 over 1,400 COAs have been issued.[14]

New FAA regulations regarding small helicopters and UAVs are due in 2015.[14] It will include new standards for registration, licensing, pilots, onboard sensors, and collision avoidance.[14]

Considering this and other types of applications that can reach quite high altitudes, it is also important for consumers to familiarize themselves with safety issues. Current guidelines for model aircraft in the United States rely on "see and avoid" as the primary means to avoid collisions.[15]

'Model' helicopters can be dangerous. Safety precautions, proper maintenance, and an understanding of the mechanics and flight characteristics of the models are necessary to prevent accidents.[16] Modelers who fly at sanctioned sites are required to follow safety rules designated by national model aircraft organizations. In the United States, the Academy of Model Aeronautics (AMA) publishes and updates safety rules for all model aircraft operations, including fixed and rotary wing models.[17] In 2014, several organizations with interest in unmanned aircraft systems in partnership with the Federal Aviation Administration launched a new educational campaign to promote safe and responsible flying and provide guidance for hobbyists and commercial users.[18]

A September 2013 incident in New York highlighted the possible dangers of remote controlled model helicopters when a 19-year-old enthusiast, who was very experienced in flying remote controlled helicopters, died after one of his helicopters' blades struck his head.[19]

In July 2013, a 41 year old Swiss man was found dead in Mauensee near his model helicopter. He had "severe head and arm injuries".[20]

Miniature helicopters are remotely controlled helicopters with a weight ranging from just a few grams to one hundred grams. Most in production are toys aimed at hobbyists and enthusiasts. In addition, there are many companies making prototypes for military and security applications. Miniature helicopters are popular demonstrations for the latest technologies in miniaturization.[21]